The present invention relates to a unistage tire building drum.
Tires comprising a two-bead carcass are normally produced on a drum comprising a cylindrical central body, and two half-drums located on opposites sides of the central body and moved in opposite directions along a common axis by a central actuating screw. Each half-drum normally comprises a ring of sectors, which, by means of a normally pneumatic actuator, are moved substantially radially with respect to the drum axis to and from an expanded position to clamp a respective bead in a given axial position along the respective half-drum. Each half-drum also comprises at least one bladder, which lies flat on the half-drum, on the outside of the bead clamping portion, and is inflatable to turn a lateral portion of a body ply up about the respective bead.
Experience has shown that, to effectively compact the material at the bead portions of the carcass being formed, each of the lateral portions of the body ply must be turned up onto an axial shoulder located on the drum just axially inwards of the portion supporting the relative bead.
Patent EP 0 459 728 relates to a unistage drum substantially of the above type, and which, in place of the cylindrical central body, features a ring of sectors located at the free ends of respective arms hinged to each half-drum and which, at rest, are located between the two half-drums and substantially parallel to the drum axis. When the central screw of the drum is activated, i.e. during formation of the carcass, the arms are rotated outwards so that the respective sectors define, on each half-drum, a ring or annular shoulder onto which the relative lateral portion of the body ply is turned up.
The above known drum has several drawbacks owing to the way in which the annular shoulders are defined. To begin with, the arms are so structured and operated that turn-up and formation of the carcass are performed substantially simultaneously, so that any additional strip of material must be applied and stitched to the already-formed carcass, and therefore with considerable difficulty. Secondly, the turn-up operation is performed against shoulders defined by hinged arms, which are intrinsically incapable of withstanding relatively severe axial pressure on the free ends. In other words, on a d rum of the type described in the above patent, using relatively high-pressure bladders and, hence, effectively compacting the bead portions of the carcass are practically impossible.
Moreover, substituting movable arms for the cylindrical central body, as on the drum described in the above patent, poses serious problems when first feeding the body plies onto the drum.
It is an object of the present invention to provide a unistage drum designed to eliminate the aforementioned drawbacks.
According to the present invention, there is provided a unistage drum for building tires comprising a carcass having two beads; the drum comprising a cylindrical central body, and two half-drums located on opposite sides of the central body and moved in opposite directions along a common axis by central actuating means; each half-drum comprising a ring of sectors, and first actuating means for moving said sectors in a substantially radial direction with respect to said axis and to and from an expanded position to clamp a respective bead in a relative given axial position along the relative half-drum; and the drum being characterized in that each said sector comprises a first and a second block, of which said first block cooperates with a relative said bead in said expanded position, and said second block comprises a cylindrical outer surface; and second actuating means for moving each said second block, with respect to the relative first block, to and from an outer position in which each second block projects radially outwards with respect to the relative first block to define a relative portion of an annular shoulder located between the given said axial position and the relative said outer surface, and in a position adjacent to the relative given said axial position.